1. Field of the Invention
This invention relates to a camera system having an automatic focus adjusting device.
2. Related Background Art
The processing of the automatic focus adjusting device of a single-lens reflex camera is as follows.
The defocus amount of the image plane of an object and the surface of a film by a photo-taking optical system is detected, and this defocus amount is converted into the amount of movement of the lens of the photo-taking optical system to be moved until the photo-taking optical system becomes in focus (the image plane of the object coincides with the surface of the film), and the photo-taking optical system is moved by said amount of movement of the lens to thereby make the photo-taking optical system in focus.
In the above-described processing, the processing for converting the defocus amount into the amount of movement of the lens has heretofore been carried out as follows.
When the defocus amount x is found, it is converted into the amount of movement y of the lens by the following equation (1): EQU y=k.multidot.x (1)
In the equation (1), k is a conversion coefficient for converting the defocus amount into the amount of movement of the lens and is defined as follows: EQU k=w/u (2)
In the equation (2), w is the minute amount of movement of the photo-taking optical system (in the case of a zoom lens or an internal focus type lens, the minute amount of movement of the focussing lens), and u is the minute amount of movement of the image plane of the object when the photo-taking optical system is moved by the minute amount of movement w.
In FIG. 13 of the accompanying drawings, there is shown the relation between the minute amount of movement w of the lens 2 and the minute amount of movement u of the image plane when the distance between the object O and the lens 2 is b and the distance between the lens and the image plane I is a.
Usually, the value of w/u is considered to be constant irrespective of the distance b between the object O and the lens 2 and k is determined to a predetermined value. For example, in the case of a totally moved type lens, when the object distance is assumed as .infin. (infinity), w=u and therefore, k=l. Also in the case of a zoom lens or an internal focus type lens, the value k.sub.1 =w/u of the equation (2) when the object distance is assumed as .infin. is adopted as the conversion coefficient k.
The approximation that w/u is constant irrespective of the distance b sufficiently holds true in popular lenses, but actually, w/u is varied by focusing and zooming and therefore, in a special lens such as a macro lens, the error of said approximation becomes great at a close distance.
In order to solve the above-noted problem, the applicant has proposed in Japanese Laid-Open Patent Application No. 62-170924 (corresponding U.S. application Ser. No. 245,967 filed on Sept. 16, 1988, now U.S. Pat. No. 4,841,325, issued June 20, 1989) the following equation (3) as the processing for converting the defocus amount x into the amount of movement of the lens: ##EQU1##
In the equation (3), the coefficients k and l are predetermined constants, and are varied by the focusing and zooming of the photo-taking optical system. For example, in a zoom lens, the zooming position of a magnification changing lens is divided into four zoom zones ZZ1-ZZ4 and the focusing position of a focusing lens is divided into five focusing zones FZ1-FZ5, and the current zoom zone ZZn and focusing zone FZm are discriminated by a zone discriminating encoder and further, coefficients kmn and lmn conforming to the zoom zone ZZn and the focusing zone FZm are determined as shown in the table below, and the determined kmn and lmn and the defocus amount x are substituted into the equation (3), whereby the amount of movement y of the lens can be found. In this table, as the number of ZZ becomes greater, the focal length becomes greater, and as the number of FZ becomes greater, infinity is approached.
TABLE ______________________________________ ZZ FZ 1 2 3 4 ______________________________________ 1 k11 k12 k13 k14 l11 l12 l13 l14 2 k21 k22 k23 k24 l21 l22 l23 l24 3 k31 k32 k33 k34 l31 l32 l33 l34 4 k41 k42 k43 k44 l41 l42 l43 l44 5 k51 k52 k53 k54 l51 l52 l53 l54 ______________________________________
If this is done, the error of the amount of movement y of the lens becomes small as compared with the equation (1).
However, in a system for finding the amount of movement y of the lens by the use of the equation (3), if the number of focusing zones and the number of zooming zones are increased in an attempt to further enhance accuracy, the scale of the encoder for zone discrimination has become large and complex.
The table of the coefficients k and l also has become great and a memory for memorizing the table also has become great in capacity.
Also, in order to avoid the above-noted problems, it is possible to calculate and find the amount of movement y of the lens directly from the defocus amount x and the parameters of the optical system such as the lens position and the focal length, on the basis of the formula of the lens, but it has been difficult in terms of both space and cost to contain means for detecting the absolute position of the lens highly accurately in a camera body or in a lens barrel.